The plant disease gray mold (“Botrytis”), is caused by the fungus Botrytis cinerea. This disease is commonly found on the stem, leaves and fruit of tomatoes. While Botrytis can be found in both greenhouse and field grown tomatoes, it is a more prevalent problem with greenhouse grown tomatoes. Moisture is of prime importance for Botrytis infection. The air must have a relative humidity of above 90% for germination of the pathogen (See, Sherf, A. F., et al., Vegetable Diseases and Their Control, John Wiley & Sons (1986), pgs. 645-647). Those areas in which fogs and heavy dews persist are more ideal for the development of the pathogen than areas where heavy rains are common. Id. The optimum temperature for growth of Botrytis is between 68° F. and 76° F. Normally, infection is rare above 77° F., although stored infected fruit can rot at temperatures as low as 32° F.
The older, senescent tissues of a tomato plant are usually more susceptible to attack by Botrytis than the younger tissues. Typically, the disease is associated with mature plants that have a dense canopy. Leaf lesions develop as light brown or gray, circular spots and may grow to cover the whole leaflet (See, Disease and Pests of Vegetable Crops in Canada, An Illustrated Compendium, Edited by Howard, R., et al., The Canadian Phytopathological Society, Entomological Society of Canada (1994)). Affected leaves become covered with conidiophores and conidia, and subsequently collapse and wither. Id. The fungus will grow from diseased leaves into the stem and produce dry, light brown lesions a few millimeters to several centimeters in length. Id. Lesions also form at deleafing scars on the stem. Id. The stem lesions may also be covered with a gray mold. Id. In severe cases, infection girdles the stem and kills the plant.
On green tomato fruit, a “ghost spot” typically appears and is the most common symptom of Botrytis. This “ghost spot” is typically tiny brown, often raised, necrotic spot that is surrounded by a pale halo. Id. Typically, once the fruit reaches a certain size, specifically, about 2.5 cm in diameter, the surface becomes smooth and shiny and tends to resist infection. Id. However, it is notable that the fruit can also become infected through flower parts stuck to the surface, particularly at the calyx end, which results in an irregular, brown lesion in the area of the flowering parts.
Unfortunately, the hereinbefore described “ghost spotting” can also occur on ripe fruit. Additionally, mature fruit can also be affected by a rot that starts at the calyx end. Id. Fruit can become water-soaked and soft at the point of infection. Id. The spots are irregular, up to about 3 cm in diameter and light brown to gray. Id. Rotting fruit will eventually fall from the plant.
In addition to tomato, Botrytis also affects a wide range of other vegetable crops such as asparagus and lettuce. The disease can be present on perennial plants in any geographical area and sporulation occurs when conditions become optimal (See, Compendium of Tomato Diseases, edited by Jones, et al.; APS Press (1991)). Conidia are easily windborn and can be blown from field to field. Id. Moreover, the pathogen can survive from season to season in the form of sclerotia, which develops on the woody tissues of tomato plants. Id. Also, Botrytis is a very efficient saprophyte, and organic matter in the soil can harbor it. Id. The fungus grows from the sclerotia or organic matter in the soil and can infect leaves lying on the ground. Id.
In order to discourage the development of Botrytis in greenhouse grown tomatoes, the temperature and relative humidity of the greenhouse should be closely regulated. Typically, temperatures higher than 70° F. and a humidity lower than 90% discourage Botrytis development. Additionally, at all times, some ventilation or forced air should be employed in the greenhouse as well. The use of drip irrigation or surface water is important to keep the leaves dry and to discourage the development of the pathogen.
For field grown plants, good drainage and weed control should be employed in order to minimize the amount of time that the plants are wet. Moreover, the nutrient levels of the plants should be kept high. It has been found that field grown tomatoes seem to have less infection and loss where nutrient levels, especially nitrogen, are kept high (See, Sherf, A. F., et al., Vegetable Diseases and Their Control, John Wiley & Sons (1986), pgs. 645-647).
Fungicides can also be used to control Botrytis in both greenhouse and field grown tomatoes. Examples of some fungicides that can be used include chlorothalonil (Exotherm Termil), that can be applied weekly and Dowicide A or DCNA (Botyan), either of which can be applied to tomato fruit post-harvest.
Presently, there are no commercially available tomato varieties that exhibit resistance to infection by Botrytis. Thereupon, there is presently a need in the art for new tomato varieties that possess resistance to Botrytis and which further exhibit desirable commercial characteristics.